In recent years, researchers and engineers have been advancing the adoption of silicon carbide (SiC) as a material for forming a semiconductor device in order to enable the semiconductor device, such as a transistor or a diode, to increase the withstand voltage, to decrease the loss, and to operate in a high-temperature environment. Silicon carbide is a wide-band-gap semiconductor having a wider band gap than that of silicon (Si), which has been widely used as the material for forming semiconductor devices. Consequently, by using silicon carbide, in place of silicon, as a material for forming a semiconductor device, the increase in the withstand voltage, the decrease in the on-resistance, and so on of the semiconductor device can be achieved. In addition, a semiconductor device using silicon carbide as the material is advantageous in that the worsening of the property is slight when used in a high-temperature environment in comparison with a semiconductor device using silicon as the material.
Generally, the method of producing a semiconductor device is implemented by combining a step of producing a wafer having a semiconductor layer and a step of heat-treating the wafer. More specifically, the method of producing a semiconductor device employs the following steps, for example. First, a wafer is produced by introducing impurities into a semiconductor layer formed on the substrate by ion implantation or the like. Then, the wafer is heat-treated by heating it to activate introduced impurities (an activation annealing).
When silicon carbide is used as the material for forming the semiconductor device, it is necessary to carry out the activation annealing at a temperature as high as, for example, not less than 1,600° C. When the heat treatment is performed at such a high temperature, however, the surface roughness of the wafer may increase (a phenomenon of surface roughening) or macrosteps may be formed by the coalescence of steps formed by the surface roughening (a phenomenon of step bunching). The above-described worsening of the surface condition adversely affects the property of the semiconductor device produced by using the wafer. In other words, when silicon carbide is used as the material for forming the semiconductor device, there is a problem in that the heat treatment of the wafer performed in the production process worsens the surface condition of the wafer, adversely affecting the property of the semiconductor device.
In order to solve the problem, a method has been proposed in which, first, a carbon (graphite) cap is formed on the surface of a silicon carbide wafer and then the wafer is heat-treated at 1,700° C. This method suppresses the step bunching on the surface of the wafer, thereby suppressing the worsening of the surface condition (see Nonpatent literature 1, for example).                Nonpatent literature 1: Y. Negoro et al., “Flat Surface after High-Temperature Annealing for Phosphorus-Ion Implanted 4H—SiC (0001) Using Graphite Cap,” Materials Science Forum, 2004, Vols. 457-460, pp. 933-936.        